Dispersions of hard, non-compliant particles are used in a variety of industrial processes including preparation of abrasive coatings, inks, paints, and the like. In addition, fluid dispersions of hard particles are used extensively in the coating of magnetic media, such as magnetic disks and tape for audio, video or data storage. In particular, the dispersion may contain magnetic pigment particles and a polymeric binder carried in a solvent. The coating process involves application of the dispersion to a substrate, followed by drying to remove the solvent, to form a magnetic recording medium.
For high-density magnetic media, the size and uniformity of the magnetic pigment particles is extremely important. To produce magnetic particles in a desired size range, coating processes often make use of media mills such as ball mills, sand mills or attritors. In each case, the media mill is designed to achieve acceptable magnetic dispersions by subjecting the pigment particles to high intensity microshearing. In this manner, the media mill breaks down agglomerations of the pigment particles, producing desired particle sizes. The media mill may not be completely effective, however, in reducing all of the pigment particles to the desired size range.
Filtration of coating dispersions generally is accomplished by filtering the solution through a series of restrictive filter elements. This type of filtration process is known as challenge, or dead-end filtration, where all of the solution is directed to flow through the thick wall of the filter media, i.e., “challenging” the media. The larger particles are trapped throughout the complex openings of the filter media matrix.